The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications, is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, may be used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Techniques for making microresistivity measurements of a subterranean formation are well known in the prior art for both wireline and LWD operations. For example, a conventional “four terminal” sensor configuration includes at least four electrodes: including at least one pair of spaced potential electrodes deployed between current injector and current return electrodes. In use, an alternating current is passed between the injector and return electrodes and a corresponding potential drop is measured between the potential electrodes. The potential drop tends to be approximately proportional to the resistivity of the formation opposing the potential electrodes for a given injected current. U.S. Pat. No. 6,191,588 to Chen discloses one example of a four terminal sensor configuration.
Those of skill in the art will understand that oil based (nonconductive) drilling fluid is commonly utilized when drilling through water soluble formations (e.g., including salt layers). Microresistivity logging in such nonconductive drilling fluid typically requires the use of high frequency alternating currents and voltages so as to reduce the electrical impedance of the oil based drilling fluid and enable a portion of the current to penetrate the formation. The use of high frequencies is also known to cause displacement currents in dielectric materials in the microresistivity sensor and in the non-conductive drilling fluid (those of skill in the art will appreciate that the electrical impedance of these dielectrics decreases with increasing frequency). These displacement currents exist even without any formation around the sensor, i.e., with the sensor in air or vacuum. In a subterranean borehole, the displacement currents create spurious potential drops across the potential electrodes that are not related to the formation resistivity. The displacement currents therefore tend to reduce the sensitivity of the sensor to changes in formation resistivity, particularly in highly resistive formations in which the formation signal tends to be weak. The displacement currents also tend to introduce an absolute error into the measured formation resistivity. As such, there is a need in the art for improved methods for making microresistivity measurements. In particular, there is a need for high-frequency measurement methods that account for the presence of the aforementioned displacement currents.